Modern motor vehicles are increasingly being equipped with exhaust systems having heat exchangers for the recovery of energy from the hot exhaust gas flow. To this end, the exhaust line typically has a branching-out structure with a heat exchanger branch and a bypass branch, wherein for example the exhaust gas flow can be optionally conducted via the bypass branch and/or the heat exchanger branch via an exhaust gas flap adjustably arranged in the exhaust line. The heat exchanger is connected to a cooling circuit for circulating coolant, which for example is the cooling circuit of the combustion engine.
In the case of such an exhaust system, the hot exhaust gas following a cold start is usually conducted entirely via the heat exchanger in order to recover valuable energy from the exhaust gas and to accelerate the heating-up phase of the combustion engine. In contrast with this, the hot exhaust gas is conducted at least in part through the bypass branch in the case of a warm combustion engine in order to protect the coolant from excessive heating. In addition to this, the emission characteristic of the combustion engine can be influenced through this measure in an advantageous manner.
The fundamental construction of such an exhaust system is well known to the person skilled in the art and has already been thoroughly described in the patent literatures. Merely as an example, reference is made in this connection to the German disclosure Publication DE 102008023806 A1.
The exhaust flap can be brought into different operating positions in order to conduct the hot exhaust gas to the bypass branch or the heat exchange branch in the desired manner. Up to now, the exhaust flap is actuated by means of a mechanical actuator, for example a so-called “wax motor”. This is a linear actuator, wherein a push rod coupled to the exhaust flap interacts with a wax capsule. When the wax capsule for example is heated through the surrounding coolant, the wax expands with the result that the push rod is moved further out of the actuator housing and the exhaust flap is moved in the wand direction. When the wax capsule cools down again, the push rod is again retracted in the actuator housing for example by means of a spring, so that the exhaust flap is moved in the other direction. As a rule, the wax motor is thermally coupled to the coolant of the exhaust gas heat exchanger, so that the exhaust flap is thermostatically controlled. As an alternative it would also be possible for example to provide a vacuum box instead of a wax motor, which is activated for example by the engine control unit as a function of the coolant temperature of the engine.
Thus, a thermostatic control of the exhaust flap takes place in conventional exhaust systems, where there is no utilizable information available with respect to the position of the exhaust flap. The use of an electric actuator and/or an electric sensor is out of the question because of the heat development that is present in the exhaust line.